Such boosters are well known in the art and generally comprise a casing divided in leaktight fashion by at least one moving partition structure into at least one front chamber permanently connected to a source of low pressure, and at least one rear chamber selectively connected to the front chamber or to a source of high pressure by a three-way valve means, the valve being operated by an operating rod.
The purpose of such an arrangement is to create a pressure difference across the two faces of the moving partition so as to generate on a push rod, which, for example, operates a master cylinder controlling the hydraulic pressure in the wheel cylinders of the vehicle, a boost force which adds to the operating force exerted by the driver of the vehicle on the brake pedal.
Some years ago devices were developed which made it possible to control the action of the brakes even when the driver of the vehicle was not depressing the brake pedal. Such automatic braking actions may be triggered by one or more detectors of operating parameters or parameters of the attitude of the vehicle, for example to prevent wheel-spin on vehicle acceleration, to help the driver of the vehicle to pull away on an incline, or under the control of a device of the anti-collision radar type.
Systems of this type, combined with pneumatic boosters, are described, for example, in documents EP-A-0,171,585, EP-A-0,303,470, and EP-A-0,347,583. The solutions illustrated in these documents have a number of drawbacks. Firstly, they require either an appreciable modification to the moving partition which separates the front chamber from the rear chamber, and especially to the central part of this moving partition, compared with the conventional configuration, or a completely special piston to allow atmospheric pressure to be applied directly in the rear chamber for operation in automatic braking mode.
Furthermore, during operation in automatic braking mode, the booster response time is relatively long because of significant pressure drops in the flow of air through the control solenoid valve, the central or offset air-transfer boot, the supply ducts formed in the pneumatic piston, and finally, the relatively narrow valve passage in the three-way valve. These same pressure drops are encountered in the opposite direction at the end of the automatic-braking operation, during what is conventionally known as brake release, and they can be significant enough that they cause the driver some annoyance.
Furthermore, the air-transfer boot, and more specifically its central part, is alternately pressurized/evacuated at the same time as being compressed/stretched. This repeated pressing causes rapid wear of the boot, and therefore detracts from its longevity and from the reliability of the booster.
Finally, operation in automatic braking mode is very difficult, sometimes even impossible, to obtain if the booster is not in its position of rest, that is to say if the driver of the vehicle is in the process of pressing the brake pedal.